[HN Gopher] Why the brain's connections to the body are crisscro...
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Why the brain's connections to the body are crisscrossed
Author : rolph
Score : 163 points
Date : 2023-04-22 15:17 UTC (7 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| killdevil wrote:
| I have always assumed that bilaterally symmetric animals have
| neural cross-wiring so that the two hemispheres of the brain are
| forced to cooperate more than they otherwise would. Simple
| animals are prevented from, say, hoarding resources selfishly and
| maladaptively on the right.
| Waterluvian wrote:
| Tangentially related to this is when I installed a patio in my
| yard two summers ago. Okay, let me explain:
|
| I had a sugar maple about 3 feet away from where I had to dig the
| foundation for the patio. An arborist said that it's a bit close
| but the maple is strong and it should recover fine.
|
| I cut out a bunch of roots and within a month the exact sections
| of the canopy corresponding to the roots I cut went brown.
|
| It was incredible to me how the roots relate directly to the
| canopy. I always kinda thought it was one big circulatory system,
| where everything supports everything. I expected the whole tree
| to struggle a bit.
|
| Next season the tree was fully recovered.
|
| The neurological system seems to work the same way? It's a
| directed graph where one root supports a very specific set of
| branches? I guess the circulatory system is like that too if you
| separate the two sets. Nature doesn't really like cyclic graphs,
| does it?
| ciconia wrote:
| > I cut out a bunch of roots and within a month the exact
| sections of the canopy corresponding to the roots I cut went
| brown.
|
| Root systems in general (for trees at least) mirror what's
| happening above ground. Pruning trees is a beneficial
| intervention, as it causes the corresponding roots to die and
| decompose. This not only makes precious biomatter available for
| recycling by the microorganisms in the soil, but also releases
| chemical signals that cause the tree itself and neighbouring
| plants to send out new growth.
| rolph wrote:
| i think of it like a plinko ball setup, in analog.
|
| neural convergence, and divergence, produce logic arrays that
| integrate many parameters to one integrated decision, vice
| versa
| Waterluvian wrote:
| If your model includes a tiny Bob Barker commentating on the
| transmission of neural signals, I'm on board.
| PartiallyTyped wrote:
| > Nature doesn't really like cyclic graphs, does it?
|
| I had a course in computational neuroscience as part of my
| bachelor's and one of the things that we covered was that the
| timing of fires is important, in that depending on how soon
| before or after a neighbouring neuron fires, the connection may
| be weakened or strengthened. This is called Spike-timing-
| dependent plasticity:
|
| > Under the STDP process, if an input spike to a neuron tends,
| on average, to occur immediately before that neuron's output
| spike, then that particular input is made somewhat stronger. If
| an input spike tends, on average, to occur immediately after an
| output spike, then that particular input is made somewhat
| weaker hence: "spike-timing-dependent plasticity"
|
| From [1].
|
| The implication of that, I believe, is that it prevents short
| cyclic graphs, for the sole reason of avoiding feedback loops
| that can cause the brain to go haywire (lol) due to the
| feedback loop. It sounds like an evolutionary adaptation to
| prevent short-circuiting.
|
| From Hebbian Learning, we have that the cycles would become
| easier to trigger, meaning that it is a feedback loop that
| increases efficiency, however, without a mechanism to prevent
| this cyclical feedback loop, the brain could be filled with
| cycles that eventually turn to just rings, which is probably
| not a desirable property.
|
| If anyone knows more about this please tell me. If it's a new
| idea, please remember to add my name :')
|
| [1] https://en.wikipedia.org/wiki/Spike-timing-
| dependent_plastic...
| rolph wrote:
| just keep inmind, this, and other phenomenon dont happen in
| all neurons, or in any particular neurons 100% of the time.
|
| neurons change functional, and structural state, depending on
| past events [hysterisis] and will shut down/modify state
| activities depending on feedback from post synapic neurons.
|
| also neurons will get tired and handoff activity to similar
| neurons in a cohort.
| IIAOPSW wrote:
| Its very much not a directed graph. We do have brain loops.
| That's where brain waves come from, circular paths of neurons
| activating themselves in a circular firing squad.
| gus_massa wrote:
| I still don't buy it. Once you cut a person vertically in two
| half with a guillotine, you get two topological disks of skin.
| Each disk can be mapped into the 2D surface of the brain. It
| doesn't matter if the parts of the skin are in the same plane or
| rotated 90deg.
|
| Obviously some parts of the skin are stretched, so a 2D map will
| cause a lot of deformation. Also some parts of the skin are more
| sensitive than other and will need more brain surface. But this
| is what is happening, there are a few maps in the brain
| https://en.wikipedia.org/wiki/Cortical_homunculus and they are
| quite deformed, and they even have a few cuts here and there.
|
| Once you decide to cut the map in two parts, each part can be
| projected in both orientations without geometric problems.
| [deleted]
| N659 wrote:
| There's a theory that says an ancestor of all vertebrates just
| flipped its head around and it stayed that way. (Source:
| https://arxiv.org/abs/1003.1872). Can someone who knows more
| about animal physiology explain this paper? Thanks.
| Shorel wrote:
| But that issue is related to the fact that insects have their
| nerves in the inferior part of their bodies, and the digestive
| tract on top, while we have our nerves in the back (protected
| by bones), and the digestive tract on the front, which would be
| the lower part if we were still walking with four limbs.
|
| The issue described here is left-right symmetry, and it also
| applies to insects, so it is more general.
| N659 wrote:
| So far I have read that these are related theories and the
| "somatic twist" theory is an expansion on the earlier theory
| of inversion. And there seems to be another related but
| separate theory called the axial twist theory (explained in
| the linked paper).
| jiggawatts wrote:
| This should be the top comment here: a reference to an actually
| _scientific_ analysis that uses an argument that is plausible
| from an evolutionary perspective.
|
| E.g.: someone else here was arguing that maybe the mirroring
| helps the brain keep processing inputs from the side that was
| hit. Evolution does not work this way! Flipping doesn't "just"
| happen, that's a huge morphological change. It had to have
| evolved incrementally, with each intermediate step having an
| immediate benefit.
|
| The paper explains how and why this may have occurred.
| m3kw9 wrote:
| I think it evolved to be crisscrossed, likely non crisscrossed
| ones were just eliminated, I'm guessing for redundant reasons.
| Having it cross covers a bigger area and may serve to cover
| issues if part of the other side get severed.
| renewiltord wrote:
| Fascinating. The magazine doesn't appear to have a way to pay to
| subscribe.
|
| Very interesting idea described here. I think it would benefit
| from some animations.
|
| I'm hoping that LLMs will be able to generate and animate SVGs to
| help with this.
| crazygringo wrote:
| > _Since Quanta is a nonprofit foundation-funded publication,
| all of its resources go toward producing responsible, freely
| accessible journalism that is meticulously researched,
| reported, edited, copy-edited and fact-checked. And our
| editorial independence ensures the impartiality of our science
| coverage -- our articles do not reflect or represent the views
| of the Simons Foundation._
| [deleted]
| nnlsoccer wrote:
| Odd that in the entire explanation they completely ignore
| embryology and the pathway dependence that came with evolutionary
| progression between species
| oncotic wrote:
| I disagree with the author's interpretation of this.
|
| As a neurologist myself, I was taught it was specifically to
| simplify visual processing, although there may be other theories
| but this is what I was taught. Like others commenting here, the
| way the lens works in each eye is by flipping the image onto your
| retina. If we had only one eye, there would be no issue, the
| image would appear as a continuous image, just flipped around.
| However, because we have two eyes, they both individually flip
| different fields, thus separating the continuity of the image
| horizontally. If you try drawing out various different ways to
| try and remedy this problem of binocular vision, the way nature's
| approach is quite elegant in reuniting the image as well as
| separating visual processing into left/right.
|
| To avoid a large text explanation, this is a simple diagram of
| the concept how the brain reforms the arrow.
| https://nba.uth.tmc.edu/neuroscience/s2/images/html5/s2_15_1...
|
| The way it works is by separating the left and right fields of
| each eye, and then crossing them so that the left fields goes to
| the right half of the brain and the right fields go to the left
| half of the brain. Each right/left half is now interpreted by one
| side of the brain and the image is again continuous if you draw
| it out on the brain. Of course now each side of the brain sees
| the opposite side, but we remedy this by crossing everything else
| so it plays well with visual interpretation. Now the right side
| of the brain sees, senses, and controls the left side of the body
| and vice versa.
|
| When it comes to everything else, there isn't a clear benefit for
| having processing swapped to opposite hemispheres. But visual
| processing benefits from it greatly, and so the rest of the
| nervous system goes along with it.
| thevagrant wrote:
| My theory is unlikely perhaps.
|
| Neural connections on the left side are potentially protected
| from damage to the right side of the body and vice versa.
|
| If the damage occurs to one side of the body but the brain is
| protected, would it not make for better chance of repair as the
| body heals?
| punnerud wrote:
| Just the swapping of sides in the brain, isn't there a clear
| benefit for the stress/weight of the nerves to the rest of the
| body?
|
| If right side was connected right arm the nerves would be
| pressed "out". When they cross there is less pressure in the
| spinal cord?
| elcritch wrote:
| Flipping the eyes is the same geometric reasoning the article
| discusses. The same thing applies to touch and motor control
| too.
|
| Its just a general extrapolation of the same principle. It
| would also explain why creatures without sight as well like
| many worms.
| bookofjoe wrote:
| https://news.ycombinator.com/item?id=35647341
| canjobear wrote:
| I don't find the TFA explanation convincing; it's too abstract.
|
| I like the explanation in terms of predator avoidance behavior.
| Imagine a primitive fish with eyes that can detect motion. If it
| sees something moving, usually it wants to get away from that
| thing. If you see something moving in your left eye, the best way
| to swim away is to send a signal for a muscle contraction in your
| right side, which will cause you to curl and swim to the right.
| So the best wiring is a direct connection from left eye to right
| side, and right eye to left side. The brain is built up starting
| from that kind of connection.
| ProjectArcturis wrote:
| But sensory input is also reversed. If you're touched on the
| right side of your body, it projects to the left side of your
| brain, which then projects back to the right side of your body
| again. If the goal were to minimize wire length, everything
| would stay on the right side.
| rhn_mk1 wrote:
| As much as I like Quanta Magazine, they dropped the ball on this
| one. I didn't understand anything of the summary of the paper. I
| hope they attach some images to the next piece about topology...
| amelius wrote:
| Also, it didn't really answer the "why" question.
| rhn_mk1 wrote:
| I presume it's somewhere in the incomprehensible explanation
| :)
| dandanua wrote:
| It's just for power balance, I think. Otherwise the left and
| right parts would be too independent. And this causes conflicts
| of interests.
| anonymouskimmer wrote:
| > While there are lots of solutions to this wiring problem, the
| most elegant is to have two bilaterally symmetrical systems of
| wiring between the brain and the body, with the connections from
| each side of the body crossing the midline.
|
| Wouldn't a 3-D brain mapping also solve this issue? Do we just
| have this flipped symmetry because neural nets started out
| non-3-D in simple organisms, and we have all just inherited the
| 2-D structure as our brains have grown? Were there ever 1-D
| neural networks, and if so, how did they work?
| rolph wrote:
| the problem is mirror image one, when mapping sensory field
| stimlus, to cortical field state.
|
| as far as 1-d networks are concerned, electrical excitation is
| not an absolute property of neurons. unicellular organisms
| employ variations of electrical potential to initiate,
| coordinate, and buffer functions, and future changes of state.
| anonymouskimmer wrote:
| > the problem is mirror image one, when mapping sensory field
| stimlus, to cortical field state.
|
| For 3-D mapping onto 3-D I don't understand what you're
| saying. Except for vision, for which I understand there are
| optical properties that require image flipping and inversion.
| rolph wrote:
| light bounces off objects and travels through the lens of
| the eye. when light exits the lens it is inverted, so
| things to your right are projected on the left side of the
| retina, the left retinae then communicate contralaterally
| to the right optical cortex.
|
| sensory field is the snapshot state of each neuron in a 2-d
| array. cortical field is same but it is now destined for
| processing.
|
| left and right fields differ by paralax and this difference
| is used, to construct a 3-d percept
| anonymouskimmer wrote:
| I thought by "cortical" you were referring to the
| cerebral cortex.
| rolph wrote:
| yes cortical in this case is applied to the cortical
| structure, of the optic lobes of the cerebrum, summarized
| as optical cortex
|
| field is the informatic state of all the neural elements,
| involved.
|
| note: these are terms applied in the context of
| neuroscience.
| svnt wrote:
| I also didn't follow the leap requiring animal experience to
| represent 3D phenomena in 2D mapping in brain space -- it feels
| like an oversimplification in order to force the peg to fit the
| hole.
| anonymouskimmer wrote:
| I think it has to do with the brain being built up in layers.
| Even though our brains are 3-D in shape, the layers are
| effectively thick 2-D surfaces. This is just a guess though.
| rolph wrote:
| you have the proper concept
| amelius wrote:
| If this is true, then perhaps we should wire motherboards in a
| way such that the connections are crisscrossed too.
| rolph wrote:
| this may be effective, when the time comes to allow a
| synthetic, to have a mobile body, that must model its sensory
| input to a common format to parse against a stored data array,
| in order to decide what scheme to employ when negotiating a
| physical, 3+d world using 2-d arrays of neuromime excitation.
| xwdv wrote:
| Will you get different thoughts and ideas if you read a book with
| one eye closed vs the other?
| kirse wrote:
| That is such a wild idea. I wonder if you dance with one eye
| covered or learn a language solely thru input from right vs.
| left eye (and ears) would it matter? Or what about musicians
| like Jimi Hendrix who played their instrument backwards?
|
| It seems un-testable in terms of eliminating any sort of
| placebo/expectation effect though.
| sgtnoodle wrote:
| Various corrective surgeries for vision over the years have
| intentionally mismatched focal distance between eyes.
| Presumably there are at least tens of thousands of people
| that can only read up close with one eye, and at a distance
| with the other.
|
| I personally was born with a crossed eye. It's been
| corrected, but the reading acuity of my secondary eye is
| worse than my primary. Everything is in focus optically, but
| reading is more strenuous. It's almost as if there's some
| letter or syllable sized gaps my brain is interpolating
| around. Perhaps I only notice it when reading because of the
| density of high frequency content in all the letters.
| eimrine wrote:
| Too slow connection between the secondary eye and letter
| recognition center?
| gibolt wrote:
| This is a mind blowing question. If so, does that mean your
| dominant eye is the default path?
| thulle wrote:
| I read some brain encyclopedia ages ago, so details are a bit
| foggy, but when the connections between the two hemispheres of
| the brain gets injured/damaged it can have some really strange
| effects. In the book they retold tests where they did things
| like showing a picture of a shovel to only one eye and ask the
| person to think of a garden tool, and they'd say shovel. Then
| they'd ask the person to explain why they're thinking of a
| shovel, and they'd tell some elaborate story like having met
| their neighbour a few days ago who said that they need to buy a
| shovel, or something similar. This due to the connection
| between the part(s) of the brain that constructs the story
| having severed connections to the parts the receives and
| processes the visual input of that eye. There was a few hundred
| pages of tests like these with wildly different effects
| depending on where the connections had been damaged.
|
| Without injury the input should be shared just fine, but since
| the difference is so severe when the connections are damaged I
| guess (with no credentials or so to back it up) that there
| could be some hard-to-measure differences in thoughts formed
| depending on which eye is used.
| callesgg wrote:
| An intressting thing with that, is that the side of the brain
| that controls speech might be completly accurate. It could
| have said showel cause that side of the brain thoght of its
| neighbors shovel. The only thing that it would be missing
| would be that it was influenced to think of that shovel event
| by input given to the other brain half.
| shagie wrote:
| A severed corpus callosum - sometimes done to mitigate the
| effects of sever epilepsy (the seizure on one side of the
| brain can't travel to the other side).
|
| Scientific American Frontiers : severed corpus callosum
| https://youtu.be/lfGwsAdS9Dc
| loa_in_ wrote:
| So it seems the answer would be yes, but we can't say how or
| to what extent.
| carapace wrote:
| I do.
|
| Try it, YMMV.
|
| - - - -
|
| Normally when you look into your own eyes in a mirror they are
| each looking into themselves.
|
| If you cross your eyes and get the distance just right, you can
| look into each eye from the other.
|
| For me it causes a strange effect, or seems to. I would be
| interested to hear reports from others of their subjective
| experience of doing that?
|
| - - - -
|
| Edit to add: Reading aloud also has different effects than
| reading silently. I surmise that the extra feedback loop from
| voice to ear has something to do with it.
|
| A common proofreading trick is to read your writing aloud. You
| catch errors that are [negative hallucination?] elided by, uh,
| non-external loops.
| svnt wrote:
| Your auditory memory is a different space than other forms of
| memory, so when you read aloud to yourself it permits you to
| buffer additional information and get to it later, in
| addition to mapping it through different perceptual space as
| you've noted. So at least the perceptual circuit and the time
| constraints are changed by doing so.
|
| I find that it no longer works very well for me though: I
| often read aloud without experiencing or remembering anything
| I've read at all. It's the same autopilot that takes over
| driving.
| loa_in_ wrote:
| Both may be true!
| skirmish wrote:
| A life hack I found useful: if I need to remember a few
| numbers for a few minutes before I can write them down, I
| keep quietly repeating them in my mind. It works much
| better for me than trying to memorize them since I think
| it uses auditory memory; after a few repetitions the
| sequence of words becomes automatic and easy to repeat
| without thinking.
| lookdangerous wrote:
| I have noticed my right eye twitch after working on the
| computer too long. I felt like my left brain had been dominant
| for too long.
| loa_in_ wrote:
| Be wary, in my personal case it was neck tightness
| restricting blood flow to the face and jaw. Normally you
| should be able to make a square by touching fingers to your
| opposing shoulders and touch wrists to your nose with no pain
| or tightness, I could not.
| rolph wrote:
| the contralateral "wiring" is also conserved in the retinal
| processing.
|
| left field of retina in both eyes communicates contralaterally
| with right hemispheric optic cortex, vice versa.
| gus_massa wrote:
| I agree, eyes are even more weird. There is a nice image here
| showing how they are conected https://www.quora.com/Which-
| side-of-the-brain-does-the-optic...
| rolph wrote:
| you may be interested:
|
| https://en.wikipedia.org/wiki/Evolution_of_the_eye
|
| https://academic.oup.com/jhered/article/96/3/171/2187545
| JonathonW wrote:
| Not in a healthy human.
|
| Cutting (or partially severing) the corpus callosum connecting
| the hemispheres of the brain has been shown to have some
| interesting results along those lines, though:
| https://en.wikipedia.org/wiki/Split-brain
| euroderf wrote:
| I recall reading that cannabis use was supposed to inhibit
| this communication between the hemispheres via the corpus
| callosum.
| modzu wrote:
| yes, if you severed the corpus collosum
| eimrine wrote:
| No if chiasm is still there.
| [deleted]
| cratermoon wrote:
| "Some of the most famous examples of confabulation come "split-
| brain" patients, whose left and right brain hemispheres have
| been surgically disconnected for medical treatment.
| Neuroscientists have devised clever experiments in which
| information is provided to the right hemisphere (for instance,
| pictures of naked people), causing a change in behavior
| (embarrassed giggling). Split-brain individuals are then asked
| to explain their behavior verbally, which relies on the left
| hemisphere. Realizing that their body is laughing, but unaware
| of the nude images, the left hemisphere will confabulate an
| excuse for the body's behavior ("I keep laughing because you
| ask such funny questions, Doc!")."
| https://www.edge.org/response-detail/11513
| turtledragonfly wrote:
| It is a fascinating series of experiments.
|
| For others interested, look up Roger Sperry's split-brain
| experiments[1], done at CalTech. He received the 1981 Nobel
| Prize for the work. I'm surprised the above article doesn't
| mention it.
|
| We don't cut cut peoples' brains in half any more, so it was
| a unique moment in time when they had people available with
| this condition, and the results are quite illuminating.
|
| Here's a video with some interviews with real patients:
| https://www.youtube.com/watch?v=aCv4K5aStdU
|
| And a here's a timestamp where one of the experiments is
| performed: https://www.youtube.com/watch?v=aCv4K5aStdU&t=101s
|
| [1] http://scihi.org/roger-wolcott-sperry-split-brain/
| d--b wrote:
| This doesn't explain why the left brain controls the right side
| of the body and vice versa, does it?
| rolph wrote:
| this would be dependency of new structure, on founding
| structure, and methods. a plan is developed early,and departure
| from that initial architechture, is a scorched earth style
| revision, versus patching old proven fixes,on new hardware.
|
| its a tendency of biological systems to innovate one step at a
| time, as persistence is a demand, so the system is not
| overhauled large scale, that would be tantamount to major
| negative selection, thus non-persistent properties.
| Acumen321 wrote:
| A theory I heard that I like, is that if you are getting attacked
| from the right, and suffer damage to the right side of your head,
| you probably want your right arm to work more than your left for
| the highest chance of fighting off the attack.
| Tagbert wrote:
| If your brain is hurt to that degree on either side, you are
| unlikely to be able to continue to fight. Evolution doesn't
| usually work to offset unusual events like this. It is more
| about optimizing operations on a daily basis.
| everyone wrote:
| I once read that an ancestor of ours evolved to have its head
| rotated 90' relative to its body (like a flatfish or something),
| but then later when it evolved a normally aligned head again,
| instead of twisting back 90' the way it had come it just twisted
| another 90'degrees
| makeitdouble wrote:
| > The nervous system is cross-wired [...] I asked my doctor last
| week why this should be, all I got was a shrug. So I asked
| Catherine Carr, a neuroscientist at the University of Maryland,
| College Park. "No good answer," she replied.
|
| After reading the piece, as he's proposition a potential theory
| based on mathematics and elegance of the solution, her reply
| still feels totally apt. That makes the title slightly
| misrepresent what he's trying to convey. We don't get the why,
| just a maybe.
|
| Otherwise I'm not sure 3d mapping simplicity is enough of an
| answer when the brain can also adapt to way more complicated
| configurations when receiving partial damage for instance. It
| also feels like we have very few organs that developped along the
| most simple solution
| spicyramen_ wrote:
| [dead]
| dghughes wrote:
| The recurrent laryngeal nerve too I'm not sure if it's all or
| most or some animals. It loops around the heart like a noob PC
| builder's first build and poor cable management. Giraffes have
| the best most extreme example of this.
| khana wrote:
| [dead]
| kibwen wrote:
| _> Letters on your T-shirt appear reversed for the same reason
| that the name "Quanta" would appear flipped, as "Quanta" if you
| wrote it with your finger on a frosty window and then went
| outside to look at it._
|
| I'm amused at the audacity of the author for asking the webdevs
| to manually wire up a unique HTML span with a custom CSS
| transform solely to make a single word appear to be rendered as
| though seen in a mirror. :)
| pbhjpbhj wrote:
| It's the inline style is in a hidden div at the end of the
| story and within #postbody, I suspect the author did it -- it's
| part of the story content at least -- and so didn't need the
| webdevs to do anything different. It's a nice touch.
| venk12 wrote:
| I understand that 2d brain to 3d brain connection requires an
| intersection (criss-cross) I drew that on a piece of paper. But I
| didn't really get why that has to be the case with 3D brain to 3D
| brain mapping. I am able to connect them without a need for an
| intersection. Are there easier examples to understand the
| intuition behind this hypothesis?
| squillion wrote:
| After an explanation that without L/R crossing the body map in
| the brain would be flipped upside down, comes this revealing
| passage:
|
| "To make sense of the sensation [...] your brain would have to
| switch from one somatotopic map to another one with the opposite
| z-axis orientation".
|
| This a textbook example of the "Cartesian theater" fallacy. It
| assumes a little person inside the brain who has to deal with an
| image projected upside-down. Of course that doesn't make sense.
| squillion wrote:
| The pictures with the finger at the end are even more
| confusing. They conflate the mapping from body parts to
| cerebral cortex (the cortical homunculus) with the mapping from
| the homunculus to some other part of the brain were the 3D
| environment is allegedly mapped. They don't make sense to me.
| refactor_master wrote:
| Slightly related: there's a YouTube video [1] of a guy teaching
| himself how to ride a bicycle "in reverse", i.e. you turn left,
| and it goes right. It takes some practice but in the end it
| just "clicks". So now he's balancing _and_ moving completely
| opposite to perception.
|
| So you're right about the little man. I struggle to see why
| some arbitrary axis transform is "too hard".
|
| [1] https://youtu.be/MFzDaBzBlL0
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(page generated 2023-04-22 23:00 UTC)